The present invention relates to a line pressure controller for an automatic transmission for controlling the line pressure supplied so as to engage/disengage frictional engagement elements included in the automatic transmission.
An automatic transmission mounted on a vehicle comprises a torque converter, a speed change gear mechanism and a hydraulic pressure supply circuit for supplying hydraulic pressure to a plurality of frictional engagement elements such as clutches and brakes provided for the speed change gear mechanism. Thus, when the power transmission passages in the speed change gear mechanism selectively operates and switches the frictional engagement elements, a predetermined speed mode can be automatically obtained. A hydraulic pressure supply circuit is connected to the actuator of each of the above-described frictional engagement elements. The hydraulic pressure control circuit comprises a regulator valve for adjusting the discharge pressure from the oil pump to a predetermined line pressure, a manual valve for manually switching the range, a plurality of shift valves for selectively operating the plurality of frictional engagement elements by switching the hydraulic passage connected to the actuator of each of the frictional engagement elements, and variable valves for performing the auxiliary operations.
The line pressure to be supplied to each of the frictional engagement elements is set to the most suitable level in consideration of a variety of factors which will be described.
For example, the line pressure is set by the regulator valve to a level which corresponds to the output torque from the engine. Furthermore, since the engine output cannot be efficiently transmitted if the frictional engagement element encounters slip, the allowable line pressure level corresponding to the engine output torque is set in proportion to the output torque in order to prevent the slip of the frictional engagement element.
As disclosed in Japanese Patent Publication (Kokoku) No. 49-30051, the line pressure is lowered when the speed change operation performed by a driver is detected for the purpose of reducing the shock due to the speed change operation. The thus lowered pressure is restored to the original level after the speed change operation has been completed.
The time period in which the line pressure is lowered according to the structure disclosed in Japanese Patent Publication No. 49-30051 is arranged to be from the start of the speed change operation to the completion of the same. Specifically, it is arranged from the generation of the speed change command signal to a time after a predetermined period To has elapsed from the making of the determination of the completion of the speed change operation. A line pressure during the term is determined in accordance with the engine output torque and turbine revolution.
As described above, according to the conventional structure, the line pressure is conducted in a fine manner for a certain time period for the purpose of preventing the shock due to the speed change operation.
However, a driver of a vehicle tends to perform the accelerating operation in accordance with the driving condition regardless of the speed change operation performed in the automatic transmission.
The accelerating pedal may be operated in the abovedescribed predetermined time period T.sub.0 after the determination of the completion of the speed change operation to the time at which the line pressure has been restored. The above-described condition can be regarded to be that the speed change operation has been completed. However, according to the line pressure lowering control disclosed in Japanese Patent Publication No. 49-30051, the line pressure is lowered during the above-described time period To. Therefore, slip occurs in the frictional engagement element of the automatic transmission, causing a problem to arise in that the engine output cannot be properly transmitted to the wheels. At this time, although the driver operates the accelerating pedal by foot, a corresponding acceleration feeling cannot be obtained.